Field of the Invention and Related Art Statement
The present invention generally relates to a technique for detecting automatically characters, numerals and marks engraved in a side wall of tire to derive tire information such as kind, size and shape of tire.
Heretofore, in order to classify or identify various tires, an operator reads visually characters, numerals and marks provided on side walls of tires in the form of protrusion or groove, tread patterns or color marks. Hereinafter, these characters, numeals and marks are called tire information marks. The number of sizes, shapes and kinds of tires has become very large in accordance with various needs of users. Therefore, the visual classification might cause human errors and labor work of the operator might be severe. Under the above circumstances it has been earnestly desired to develop a method of automatically reading the tire information marks. There have been proposed various methods for detecting automatically the tire information marks. In one method disclosed in Japanese Patent Application Laid-open Publication No. 63,843/75, a side wall of tire is illuminated and is optically picked up by a camera head including imaging lens and image sensor, while the tire and camera head are relatively rotated at a constant speed. An image signal supplied from the image sensor is suitably processed to detect the marks. It has been also proposed to provide a bar code of crenelated construction on the side wall of tire and the bar code is mechanically read by means of a bar code reader comprising a limit switch. Further it is also known to apply a label having a bar code printed thereon on the tire and to read the bar code optically. It is further proposed to print marks directly on the tire surface and to read the marks optically. It is also known to record marks on the tire surface with a fluorescent ink and to read the marks by illuminating them with the aid of a fluorescent lamp.
However, all the above explained methods are not satisfactory and have serious drawbacks in the practical view point. For instance, in case of detecting optically the tire information marks engraved on the side wall of tire, it is very difficult to extract only the desired tire information marks, because there are formed on the side wall of tire various characters and marks other than the tire information marks and these undesired characters and marks might cause noise. It should be noted that a position of the tire information marks on tire viewed in a radial direction differs for various kinds of tires and a height of the side wall is also varied in accordance with a width of tire. Moreover, since the tire information marks have the same color as that of a background, an image of the side wall projected upon the image sensor has a very low contrast. Therefore, a photoelectrically converted signal has a very small signal-to-noise ratio.
In the method of reading optically the engraved bar code, since the operator could not comprehend the bar code, it is very inconvenient. Further, the mechanical bar code reader might be worn soon. In case of using the bar code label, it is quite cumbersome to apply labels onto respective tires. In the known method of printing the marks on the tire, when the printing is effected prior to the vulcanization, the marks might be degraded during the valcanization, and even if the printing is carried out after the vulcanization, the printing operation is troublesome and human error might be introduced. When the special ink is used, the printing operation is further cumbersome, and further the special reading apparatus is required.
The inventors have stuided and analyzed the various methods and found that it is the best to form the tire information marks during the vulcanization and to detect optically the thus formed marks.
In the known method of reading optically the tire information marks which are molded in the side wall of tire during the vulcanization, the side wall of tire is illuminated vertically from a direction which is perpendicular to the surface of side wall and an image of the side wall surface is picked up by a television camera. In such a method, when the tire surface has a high reflectance, light reflected by an inclined side of mark is not substantially made incident upon the camera, but light reflected by a background is almost made incident upon the camera, so that it would be possible to obtain an image of the tire side wall surface having a high contrast. However, in general, the side wall of tire has a very low reflectance and has a scattering property. Therefore, light reflected or scattered by the marks is partially made incident upon the camera and almost all light reflected by the background does not impinge upon the camera. Therefore the contrast of the picked-up image is very low and the mark could not be detected accurately. It is also possible to illuminate the side wall of tire with light from a direction which is inclined or oblique with respect to the surface of side wall and to pick up an image of the side wall from a direction perpendicular to the surface of side wall. Also in this case, the contrast between the mark and background is low and it is difficult to read the marks accurately.